1. Field of the Invention
The present invention relates to a screw type extruder for melting solid polymer materials. More specifically, the invention relates to a screw extruder that transforms solid polymer materials to a melted fluid material by contact of the solid polymer materials with a heated surface. The heated surface is provided with a plurality of circumferentially-spaced, axially extending, radial slots such that the fluid material passes outwardly or inwardly through the radial slots and is removed from the heated surface to ensure that the heated surface remains in close contact with the unmelted solid polymer material at all times for improved heat transfer.
2. Related Art
Devices for converting solid thermoplastic materials to a melted condition are well known in the art. Known devices for use in manufacturing plastic products utilize, in one form or another, an extrusion device to melt, mix, and pump molten polymer for further processing. A commonly used extruder is known as a single screw extruder. A typical extruder mechanism of the single screw extruder type utilizes a heated barrel, a helical auger or plasticating screw within the barrel and a feed throat. The screw is rotated by conventional means. The melting of the plastic is accomplished by heat conduction and shear energy dissipation at the inside wall of the barrel. The shear energy is generated as a result of relative motion between the packed solid polymer particles in the screw channel (solid bed) and the heated stationary barrel surface. As the solid polymer particles are melted, a layer of melt is formed on the barrel""s inner circumferential surface, which is continuously wiped off by the screw thread or flight. Barrier type screws are also known for use in the single screw extruder wherein the external thread of the screw includes a rear side portion of full thread radius and a leading side portion of smaller radius, between which a groove is formed. The barrier screw can provide an improved means for separating the melt from the solid bed in order to increase the efficiency and quality of the melt.
In U.S. Pat. No. 3,689,182 to Kovacs, a screw extruder is shown wherein the polymer melt is bled radially inwardly into a central axial bore containing a second coaxial screw. In Kovacs, radial holes 14 of xe2x85x9 inch diameter are distributed along the leading face of the thread of the external screw at uniform angular spacings to dispose six or eight radial openings per turn of the thread. In Kovacs, the assumption is made that the polymer melt forms in a small pool along the leading side of the thread on the external screw. Hence, Kovacs states that satisfactory removal of the melt is obtained with openings of xe2x85x9 inch in diameter distributed serially along the leading face of the thread. In the embodiment shown in Kovacs employing a barrier type screw having a rearward portion of full thread radius and a leading side portion of a smaller radius of thread, Kovacs assumes that a spiral solid thermoplastic body will develop along the full length and volume of the channel defined by the thread. Melted material is only removed from the channel by radial holes positioned serially along the groove defined between the rearward and leading side portions of the thread. Thus, a continuous layer of melted polymer is allowed to form along the entire circumferential extent of the spiral channel defined by the thread, with the melt only being bled off at the leading edge of the thread.
The second embodiment of the Kovacs patent shown in FIGS. 4, 5, 6 and 7 employs a screw 30 having a thread 31 which terminates at its downstream end at a location upstream of, and spaced from, a head 33 as shown in FIG. 5 of the patent. Head 33 has a smooth perimetric area machined to form a desired clearance with the inner surface extruder barrel to restrain the passage of material through said clearance except for that necessary to prevent fouling as discussed in the paragraph beginning in Column 5, Line 55 of Kovacs. The head 33 has grooves 57 which are closed at their front ends and open only at their rear ends with the purpose of the grooves being to trap solid particles. Head 33 also has additional grooves 58 which are closed at both their front and rear ends and which collect melted material from grooves 57. Thus, the second embodiment of Kovacs does not disclose linear threads extending from or adjacent the downstream end of the thread 31 nor is such structure shown elsewhere in the Kovacs patent.
Cheng U.S. Pat. No. 5,098,267 discloses an elongated rotatable screw plunger 12 positioned for rotation and reciprocation in a barrel 10 with the screw plunger 12 extending through an upstream metering section 28 and a mixing section 40 immediately downstream of metering section 28. The screw 12 includes a bore 50 solely located in its downstream end in mixing section 40 as shown in FIG. 5. Lands 38 are provided in metering section 28 and are separated by metering section grooves 36 and a gap space is provided between the downstream end of the spiral thread of the screw and the upstream ends of lands 38.
The mixing structure immediately downstream of the lands 38 consists of mixing section grooves 42 provided between a xe2x80x9clandxe2x80x9d 44 which would better be described as a barrier plate or flight which has a reduced height relative to the general surface of plunger 12 and a xe2x80x9clandxe2x80x9d 46 which could better be described as a wiper plate since it is adjacent the inner surface of the barrel 12. Rotation of the plunger 12 causes plasticate to flow across xe2x80x9clandxe2x80x9d 44 and then downwardly into slot 48 because of the shape and dimension of land 46 as discussed in the paragraph beginning in column 4, line 26 of the Cheng patent.
Soviet Union publication SU 1763207 discloses a screw having spiral threads and an internal bore but not having linear threads spaced from the spiral threads or being closely spaced from the inner surface of the barrel in which the screw is mounted.
In prior art screw-type extruders, such as Kovacs and others, the thickness of the melt film formed along the inside of the heated barrel directly affects the efficiency and rate of melting since the heat developed by dissipation of shear energy between the solid polymer and the barrel is reduced. This reduction in melting rate is especially important on large extruders wherein the circumferential length of the solid bed is very large.
The present invention provides a device for transforming solid thermoplastic materials into fluid material by contact with a surface, wherein melting of the thermoplastic material is caused by heat conduction and shear energy dissipation. The shear energy is created by relative motion between the solid polymer material and a surface, and the surface is provided with a plurality of circumferentially spaced, axially extending, radial slots there through such that melted polymer material can pass through the surface into an area for storage or removal. The provision of circumferentially spaced, axially extending, radial slots allows the surface to remain in close contact with the solid material at all times by providing means for draining off melted polymer at several locations around the circumference. Hence, the layer of melt interfaced between the solid polymer material and the surface is kept very thin, thus improving heat transfer and shear energy dissipation.
In one embodiment, the present invention is provided with an external barrel that is heated by heating coils, an internal screw, and a floating cylindrical sleeve having a plurality of circumferentially spaced, axially extending, radial slots there through located between the external barrel and the internal screw. The internal screw comprises a central core and an external thread so interrelated in cooperation with the floating cylindrical sleeve as to define a continuous spiral channel between the screw core and the floating cylindrical sleeve. The portion of the spiral channel located radially inwardly from the heating coils constitutes a melt section that includes a substantial portion of the length of the channel and that terminates adjacent the front or material discharging end of the screw. The thread and the core of the screw can be shaped through the melt section proceeding in the direction of material advancement to progressively decrease the depth of the spiral channel without substantial change in its width. As a result there is a corresponding progressive decrease in the cross sectional area of solid polymer material contained in the spiral channel is it advances toward the discharge end of the screw. The coaxial sleeve fits between the external barrel and the internal screw. The coaxial sleeve has helical flights or threads cut into its outer circumferential surface, and circumferentially spaced axially extending radial slots cut through to the inside circumferential surface of the coaxial sleeve.
The floating cylindrical sleeve is coaxial with the inner screw and the external barrel. The sleeve is constrained from axial movement, but is free to rotate. The speed of rotation of the sleeve is a function of the radial clearances between the sleeve and the inner screw and between the sleeve and the external barrel. The shear stress generated by the solid polymer material and melted material contacting the inner circumferential surface of the floating sleeve causes the sleeve to rotate in the same direction as the inner screw. Shear stress is also developed in the melted polymer that has passed through the floating sleeve at the interface between the melted polymer and the external barrel. This shear stress retards rotation of the floating sleeve to a speed less than that of the inner screw. The difference in rotational velocity between the inner screw and the coaxial sleeve generates the shear stress and resultant heat energy that helps to melt the polymer at the outer diameter of the internal screw. The axially extending, radial slots through the cylindrical floating sleeve are spaced around the circumference of the sleeve such that the circumferential distance between the slots is considerably less than the circumferential distance of melt film formed on conventional or even barrier type extruder screws.
In one embodiment of the present invention, the external barrel is provided with internal threads that serve to wipe the melt film off the external circumferential surface of the cylindrical floating sleeve and move the melt to the discharge end of the extruder screw. Alternatively, the cylindrical floating sleeve is provided with external threads and the barrel is provided with a smooth internal surface. A further embodiment comprises a cylindrical floating sleeve with a smooth external surface, a surrounding barrel with a smooth internal surface and a separate spiral member located between the floating sleeve and the barrel defining a channel along which the melted polymer is moved toward the discharge end of the extruder. Additional embodiments can include means for rotating either the barrel, the cylindrical floating sleeve, and/or the separate spiral member in order to provide the advantage of more complete control over the relative rotational velocities of the parts of the extruder screw for optimum performance.
A further embodiment of the present invention comprises an external barrel and a coaxial internal screw having external threads that fit closely to the inner circumferential surface of the barrel, wherein the internal screw is provided with an internal axial bore for the removal of the melted polymer material. The internal screw is also provided with multiple external threads such that a plurality of continuous spiral channels are formed along the external surface of the screw to move the solid polymer materials along the screw. In this embodiment, the screw flights or threads extend axially substantially parallel to the screw axis over a substantial length of the melt section in the extruder screw. Axially extending radial slots through the screw are provided at the leading edge of the axial screw flights or threads, providing passageways into the center axial bore through the screw.
The circumferential spacing of the radial slots through the internal screw is determined by the number of axial threads provided on the screw. The pressure created by the flow of melted polymer into the center axial bore of the screw forces the melt along the axial extent of the bore until it exits from the discharge end of the extruder screw. The force to keep the solid polymer material moving along the axial threads in the melt section of the extruder screw is provided by the beginnings of the threads in the feed section which are provided with a helix angle.
In a further embodiment of the present invention, the internal screw can be provided with a plurality of internal axial bores providing multiple passageways for movement of the polymer melt to the discharge end of the extruder screw. These multiple internal axial bores are arranged in a circular pattern a short distance below the surface of the screw such that the depth of the radial slots passing from the screw channel to an associated internal axial bore is kept short. The number of screw threads provided along the internal screw equals the number of internal axial bores for moving polymer melt to the discharge end of the extruder screw. The radial slots pass from the leading edge of each screw thread radially inwardly to an associated internal axial bore.